The present disclosure relates to a noise reduction apparatus and method, and a program, and more specifically, the present disclosure relates to a noise reduction apparatus and method, and a program which make it possible to reduce discomfort due to residual noise in noise cancelling.
In related art, noise cancelling headphones have been developed which lower noise level at the listening point by outputting a signal that cancels out noise from the surroundings.
Noise cancelling techniques adopted in noise cancelling headphones currently in practical use fall roughly into two schemes, feedback and feedforward schemes.
Generally, feedback noise cancelling systems have a characteristic that although the band of frequencies over which noise can be cancelled (the band over which noise can be reduced) is narrow, relatively large reduction is possible.
On the other hand, although feedforward noise cancelling systems can reduce noise over a wide band and have stability, there is a conceivable possibility that in the event of disagreement with the assumed transfer function due to factors such as relative position to the noise source, noise increases at the corresponding frequencies.
For this reason, in cases where feedforward noise cancelling systems which can cancel noise over a wide band and have stability are used, there are times when noise becomes conspicuous within a specific narrow band, even though noise is reduced over a wide band. In such cases, the listener (user) is not able to perceive the reduction effect.
As a measure to overcome this problem, there have been also developed techniques that operate also the feedforward scheme simultaneously in addition to the feedback scheme, thereby providing a greater noise reduction effect.
However, in related art, the filter characteristic is constant (fixed filtering scheme) in both the feedback and feedforward schemes, with the noise cancelling characteristic remaining constant even with a change in noise. For this reason, strictly considered, there may be cases where it is not possible to achieve optimum noise cancelling performance for each type of noise. For this reason, use of the adaptive filtering scheme is conceivable as a way to achieve optimum noise cancelling performance for each type of noise.
Accordingly, the applicant has proposed a noise cancelling technique that combines the fixed filtering scheme and the adaptive filtering scheme (see, for example, Japanese Unexamined Patent Application Publication No. 2010-4250).